https://www.amazon.co.uk/gp/product/B06XXMRZ2BSomething cheap like that works fine - just a paperclip with a switch basically.
As for turning it on, as long as you turn on the second psu first - then power on the system/sys psu Your fine.
1second - ?? no don't need to worry about length of time between powering things on.
As long as you only have GPU's and their risers powered from the second one.